dark matter

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Hypothetical matter that, according to certain modern theories of cosmology, is thought to make up over 90% of the mass of the universe but so far remains undetected. Measurements of the mass of galaxies using modern theories showed large discrepancies in the expected values, which led scientists to the conclusion that a theoretical substance that cannot be seen had to account for a significant proportion of the universe. Dark matter, if shown to exist, would account for many currently unexplained gravitational effects in the movement of galaxies.

Theories of the composition of dark matter include unknown atomic particles (cold dark matter) or fast-moving neutrinos (hot dark matter) or a combination of both. Other theories postulate that massively dense objects such as black holes form the majority of the ‘missing’ mass in the universe.

In 1993, astronomers identified part of the dark matter in the form of stray planets and brown dwarfs, and, possibly, stars that have failed to ignite. These objects are known as massive astrophysical compact halo objects (MACHOs) and may make up approximately half of the dark matter in the Milky Way's halo.

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